1. Field of the Invention
This invention relates to a multilayer circuit board in which not only wiring conductor but also resistor and capacitor are multilayered, as well as to a process for manufacturing said multilayer circuit board.
2. Brief Description of the Prior Art
In general electronic circuits, IC sealed with ceramics or the like, or so-called sealed IC, are used. If a non-sealed IC is used in stead of sealed IC, an IC-connecting conductor having a conductor-conductor distance of about 200 .mu.m must be formed on a substrate because non-sealed IC is smaller than sealed IC and the distance between connecting terminals is shorter in non-sealed IC than in sealed IC.
However, if the connecting conductors are formed by a so-called dry process which comprises printing a conductor paste on a sintered ceramic substrate (hereinafter, simply referred to as a "ceramic substrate") followed by drying and firing it, short-circuit takes place between the wiring conductors unless the conductor-conductor distance is as large as about 400 .mu.m, because such dry process causes blotting in printing.
On the contrary, according to the so-called wet process which comprises printing a conductor paste on an unsintered ceramic substrate (hereinafter, simply referred to as a "green sheet") composed mainly of alumina, followed by drying and sintering it, a solvent in the printed conductor paste permeates into the green sheet, so that no blotting takes place in printing. In addition, the green sheet shrinks by 10-20% upon sintering. Therefore, fine wiring conductors having high density can be formed.
However, since the sintering of a green sheet composed mainly of alumina progresses at a temperature of 1,500.degree.-1,600.degree. C., the wiring conductor must be formed of a refractory metal not changing its quality upon sintering, such as molybdenum, tungsten, manganese or the like. Further, if resistors and dielectrics are provided in the inner layer of a multilayer circuit board, they change their quality at the time of sintering the green sheet. Therefore, in the multilayer circuit board, it is necessary to form the resistors and dielectrics in the uppermost layer after sintering the green sheet.
If a green sheet can be sintered at a temperature below 1,000.degree. C., substances other than the above-mentioned ones can be used as wiring conductors, and resistors and dielectrics can be provided in the inner layer of the ceramic multilayer wiring board.
As inorganic insulating materials for the green sheet capable of being sintered at a temperature below 1,000.degree. C., there are known a crystallizable glass consisting of .beta.-spodumeme as a main component and lithium metasilicate as a subsidiary component, and a crystallizable glass consisting of .alpha.-cordierite as a main component and clinoenstatite as a subsidiary component.
However, these materials must be heated for too long a period of time (if the time period of heating was short, the peeling of wiring conductor took place) because they required a heating rate of 2.degree. C./minute or less, a retention time of 1-5 hours at a sintering temperature, and a cooling rate of 4.degree. C./minute or less.